This invention relates to intrusion systems in general and more particularly to an intrusion detection system which utilizes a plurality of vibration sensitive sensors arranged in a terminated loop.
The prior art is replete with a great many patents which essentially direct themselves to the detection of vibrations and convert the vibrations to an output signal to indicate an alarm condition. As is well known, in protecting a premises against unauthorized entry, the prior art employed various types of vibration sensitive detectors. These devices are placed and positioned on windows, doors, walls and so on and will produce an output upon a vibration of the structure exceeding a predetermined value. The output of such devices is normally detected by means of a processing circuit or other logic which enables a relatively reliable operation in that spurious vibrations or low magnitude vibrations are not responded to due to the protection circuitry.
A device which operates in such a manner is fully described in U.S. Pat. No. 4,361,740 entitled SEISMIC SENSOR APPARATUS, issued on Nov. 30, 1982 to R. Stockdale and assigned to the assignee herein. Essentially, the patent shows a sensor which employs an annular member having an interior conductive periphery. The annular member is coaxially positioned about a center post located in the housing. The interior periphery of the annular member engages two contacts, and when a vibration occurs, the annular member moves off the contacts to develop an output signal. This signal is detected to indicate an alarm by means of processing circuitry.
As indicated in the patent, there are many other devices in the prior art which also operate to detect vibrations and are widely utilized in intrusion alarm systems. In such an intrusion alarm system various sensors are normally connected in a series loop, and if one of the sensors or locations is vibrated, the loop is opened and the open circuit is detected by a processor or other control circuit to sound an alarm. There are, of course, many problems associated with this type of loop in that an intruder could conceivably short the loop at the control panel and hence prevent an alarm when he thereafter enters the premises.
There are other conditions which adversely affect the operation of such systems. It is, therefore, an object of the present invention to provide an intrusion system which employs vibration detectors in a series loop and which employs reliable processing circuitry in order to determine and indicate an alarm condition.